1. Field of the Invention
The present invention relates to an on-load tap changer having vacuum type switches.
2. Description of the Prior Art
A contact mechanism of a change-over switch which opens and closes in an insulating oil has heretofore been generally used for on-load tap changers. With this mechanism, however, the insulating oil is subject to contamination by the contacts which open and close. Recently, therefore, an on-load tap changer has been put into practice by using vacuum switches which have a contact mechanism and which do not use insulating oil, as current switching elements of the change-over switch.
FIG. 1 shows a circuit of a conventional on-load tap changer which employs the above-mentioned vacuum switches, and FIG. 2 shows a switching sequence of the contacts.
In FIG. 1, reference numeral 1 denotes a vacuum switch for a main contact on the side of odd-numbered taps, 2 denotes a vacuum switch for a resistance contact, 3 denotes a vacuum switch for a main contact on the side of even-numbered taps, 4 denotes a current-limiting resistor, 5 denotes a tap winding of a transformer, 6 denotes a tap selector on the side of even number, and 7 denotes a tap selector on the side of odd number.
The circuit of FIG. 1 is of the resistance switching system which provides a great advantage when it is used as an on-load tap changer. Further, this system features very simple sequence and operation as shown in FIG. 2. Namely, FIG. 2 illustrates a sequence for changing an odd tap number into an even tap number. On the left side of FIG. 2, the main switch 1 on the side of odd number and the switch 2 for resistance are conductive, and the main switch 3 on the side of even number is nonconductive. To change the tap, first the switch 2 is left on while, the switch 1 is turned off. Then, the switch 3 is turned on and the switch 2 is turned off. From the standpoint of simple construction and small size, this on-load tap changer pertains to a one-resistance system (per phase) which is said to be suited for the on-load tap changer having vacuum type switches. This device, however, has defects as mentioned herebelow.
(a) The breaking duty (breaking current x recovery voltage) of the vacuum switch 3 for the main contact on the side of even-numbered taps where a tapdifference current caused by voltage between odd-numbered taps and even-numbered taps is superposed on the load current when the taps are to be changed, becomes greater than the breaking duty of the vacuum switch 1 for the main contact on the side of odd-numbered taps. When a rated load is to be switched, the breaking duty becomes four times as great (here a current-limiting resistance=step voltage/rated current which is flowing). When a 200% overload is to be switched, the breaking duty becomes as great as nine times the breaking duty of the vacuum switch 1 for the main contact on the side of odd-numbered taps compared to the case of switching the rated load. Here, the mechanism for operating contacts of the vacuum switch is provided in relation to each of the vacuum switches. Therefore, the individual vacuum switches must have the same size. Accordingly, the size of the vacuum switches must be determined based upon the size of the vacuum switch which has a large breaking duty for switching overload. Therefore, the vacuum switches tend to become large in size, and the tap changer tends to become bulky.
(b) The gap between contacts of the vacuum switch when it is opened, is not allowed to be so increased from the standpoint of maintaining mechanical durability of a bellows which is used for the contact operation mechanism of the vacuum switch. When vacuum switches are employed for the on-load tap changer, therefore, a sufficiently large withstanding voltage is not maintained against impulses caused by lightning.